As it is, the map is useful until solar goes to a double digit percentage of total yearly generation. Any higher, and seasonal places start needing backup generation capacity that is only running in winter (or during monsoon), which does violent things to electricity markets.
This is being pursued most interestingly by Standard Thermal's "hot dirt" approach. They've updated their web page to include a few more technical details, which look to make sense economically (for example, they're putting the resistive heater(s) outside the dirt piles where it/they can be easily maintained).
The round trip efficiency (RTE) might be 40%, which is actually not bad since the input power (crude DC direct from solar modules) is quite cheap, not needing inverters or a grid connection sized to that DC power. Thermal losses are < 1%/month.
The technology would work down to a scale of about 100 kW average power, which is quite small all considered. Good for village-scale microgrids or individual small industrial enterprises. Scaling up, the average power would be similar to that of a large earthen dam of the same size, only the dirt would be used for to hold heat, not hold back water in a river, and could be sited almost anywhere. Heat retention improves as the size scales up; a dirt pile of the size of the largest dams, if it had been built and heated in the Bronze Age, would still be hot inside.
Still, adds a lot of cost and complexity onto cheap solar + 12h of chemical battery storage.
> This is being pursued most interestingly by Standard Thermal's "hot dirt" approach.
Pity they want to do only 400°C. Because once you go towards firebricks and >700°C, you can run practically all thermal chemical processes in a refinery or a polymer plant off the heat, and you can make electricity with a supercritical CO2 turbine at >50% efficiency (in a comparatively tiny package). At the cost of much harder engineering problems, of course.
But its probably worth solving those, because you can do interesting stuff with ultra low cost hot gas. Preheat the charge for a steel mill, design a Hall–Héroult aluminium smelter that keeps the electrolyte bath molten via a gas heat exchanger instead of electricity, ect.
And once you have several GW of guaranteed heat consumers, adding a couple of CO2 peaker turbines isn't such a big leap anymore.
But at high latitudes, this would require either unaffordable amounts of batteries, or so much solar that enormous amounts of it is being wasted during the summer. So why not store instead of waste that excess?
The alternative for those high latitude areas would be wind, which is available in winter. But then you have to deal with how you cover the rare dark-calm extended periods in winter, so you still need a backup supply (like e-fuels in turbines), or else batteries cheap enough to cover those (maybe a couple of weeks). Iron-air batteries might be good for that.
I'm guessing the 400 C is just because they have customers who only need that. They should be able to get to 600 C pretty easily; that's the temperature limit for cheap steel before creep becomes important. They don't want to use firebricks since the scheme depends on the storage medium being extremely cheap, which means dirt (maybe $1-2/ton if sourced locally). Seasonal storage requires extremely low cost storage media.
See e.g.: https://www.sciencedirect.com/science/article/pii/S2666352X2...
Volumetric energy density is pretty decent, and it's easy and safe to store (albeit heavy). The big appeal is that it seems feasible to convert coal plants while touching only their burner (there is a smallscale prototype running in Germany AFAIK). You could also potentially even transport the fuel (not sure how the losses look on that though since energy/weight is not very high; maybe ships or trains could still work though).
Drawback is that regenerating the iron powder needs (green) hydrogen and the cycle efficiency is low.
But this could be very helpful for countries like Germany, Poland or especially China to get dispatchable green power from former coal power stations.